This invention relates to the molecular beam deposition of semiconductor materials under ultra-high vacuum conditions, and, more particularly, to the sequential deposition of such materials on a plurality of substrates, and to unique effusion cells for use in the process.
As pointed out in a review paper by J. R. Arthur and myself (Progress in Solid-State Chemistry, Vol. 10, Part 3, pp. 157-191, Pergamon Press 1975), molecular beam epitaxy (MBE) is a term used to denote the epitaxial growth of semiconductor films by a process involving the reaction of one or more thermal molecular beams with a cyrstalline surface under ultra-high vacuum conditions. In comparison to simple vacuum-evaporation, MBE offers much improved control over the incident atomic or molecular fluxes so that differences between the sticking coefficients of the beam species can be taken into account. Use of shutter mechanisms and relatively slow growth rates (e.g., 1 .mu.m/hr.) allow rapid changing of beam species and growth of layers as thin as a monolayer (2.8 A for GaAs).
In addition, since electrically active impurities are added to the growing film by means of separate beams, the doping profile normal to the surface can be varied and controlled with a spatial resolution difficult to achieve by more conventional, faster growth techniques such as CVD and LPE.
MBE has been used to fabricate films of a variety of materials: Group III-V compounds, principally GaAs and AlGaAs, as well as Group II-VI and IV-VI materials (e.g., CdTe and PbS). Research also has extended to elemental materials such as Si but to date has met with, at best, mixed results. Molecular beam deposition is not limited, however, to epitaxial growth on single crystal substrates -- high resistivity, polycrystalline GaAs layers have been deposited on amorphous substrates such as SiO.sub.2.
In the GaAs-AlGaAs system MBE has been successfully employed to fabricate a number of device structures: IMPATT diodes, microwave mixer diodes, double heterostructure junction lasers, optical waveguides and superlattices. Fabrication took place, however, in either a research environment or a low-volume production facility so that processing of a single substrate (which can typically accommodate hundreds of devices) at a time was adequate.
It is, however, a broad object of my invention to sequentially process a plurality of substrates in an MBE apparatus suitable for relatively high volume production.